diff -up exp_log.c.orig exp_log.c
--- a/exp_log.c	2013-12-12 12:43:38.527854189 +0100
+++ b/exp_log.c	2013-12-12 12:49:26.866576387 +0100
@@ -176,7 +176,7 @@ expStdoutLog TCL_VARARGS_DEF(int,arg1)
 
     if ((!tsdPtr->logUser) && (!force_stdout) && (!tsdPtr->logAll)) return;
 
-    (void) vsprintf(bigbuf,fmt,args);
+    (void) vsnprintf(bigbuf,sizeof(bigbuf),fmt,args);
     expDiagWriteBytes(bigbuf,-1);
     if (tsdPtr->logAll || (LOGUSER && tsdPtr->logChannel)) Tcl_WriteChars(tsdPtr->logChannel,bigbuf,-1);
     if (LOGUSER) fprintf(stdout,"%s",bigbuf);
@@ -222,7 +222,7 @@ expErrorLog TCL_VARARGS_DEF(char *,arg1)
     va_list args;
 
     fmt = TCL_VARARGS_START(char *,arg1,args);
-    (void) vsprintf(bigbuf,fmt,args);
+    (void) vsnprintf(bigbuf,sizeof(bigbuf),fmt,args);
 
     expDiagWriteChars(bigbuf,-1);
     fprintf(stderr,"%s",bigbuf);
@@ -264,7 +264,7 @@ expDiagLog TCL_VARARGS_DEF(char *,arg1)
 
     fmt = TCL_VARARGS_START(char *,arg1,args);
 
-    (void) vsprintf(bigbuf,fmt,args);
+    (void) vsnprintf(bigbuf,sizeof(bigbuf),fmt,args);
 
     expDiagWriteBytes(bigbuf,-1);
     if (tsdPtr->diagToStderr) {
@@ -307,7 +307,7 @@ expPrintf TCL_VARARGS_DEF(char *,arg1)
   int len, rc;
 
   fmt = TCL_VARARGS_START(char *,arg1,args);
-  len = vsprintf(bigbuf,arg1,args);
+  len = vsnprintf(bigbuf,sizeof(bigbuf),arg1,args);
  retry:
   rc = write(2,bigbuf,len);
   if ((rc == -1) && (errno == EAGAIN)) goto retry;
